The invention concerns a process for driving a sensor with offset control where the output signal of a drive circuit is compared to a reference voltage in a comparator, with the comparison being carried out of a fixed or variable operating frequency as disclosed in DE 33 34 603.
Such processes for driving are required, for example, to be used in acceleration transducers in motor vehicles for triggering, e.g. airbags or belt-tensioning devices. An acceleration transducer measures the deceleration of the vehicle, converts this measured value into an electric signal which is then amplified and passed on to a control device. This control device will generate a trigger signal if the deceleration characteristics identified correspond to those normally found when the vehicle crashes into an obstacle. The trigger device will cause ignition circuits to be triggered electrically which, by means of a chemical reaction, effect either the sudden inflation of the airbag or the sudden tightening of safety belts. Here, several types of acceleration transducers are differentiated, e.g.; piezoelectric and piezoresistive transducers. In the case of a piezoelectric transducer, a charge is generated which will be output as an electric current, whilst in the case of the piezoresistive transducers an electric voltage is generated micromechanically.
Such processes for driving acceleration transducers must be extremely reliable as there is the risk that the devices are triggered incorrectly due to the offset draft of the sensor signal across time and temperature. When cheap components are used, especially cheap acceleration transducers, this risk is particularly high.
DE 33 34 603 C2 discloses a process for a piezoelectric acceleration transducer complete with offset compensation where, in a comparator, the output current signal of the amplifier arrangement will be compared with a reference value. This comparison will be effected within a fixed or variable time period, and, depending on the output current signal of the comparator, the output current of a power source will be increased or decreased in line with a time pulse, and this output current will then be fed into an amplifier input.
During turn-on, in order to create defined starting conditions, the counter will be set to a predefined numeric value and the time pulse generator will be set to a higher frequency for a defined time period. This causes an accelerated offset control process to occur which balances more quickly any turn-on deviations.
However, the disadvantage of this process is that it can only be used to drive piezoelectric, that is, current-controlled, arrangements. A further disadvantage results from the fact that at that moment in which the turn-on process is terminated--that is, when the switchover from the higher frequency required for the turn-on process down to the significantly lower operating frequency is effected-, a crash event cannot be detected by means of this process, or that the device may be triggered incorrectly. Thus, this process does not afford reliable protection for the occupants of a motor vehicle across the entire time period.